Peter Campbell Smith gurgled happily,
sounding much like they were saying:

I don't know the real answer either, but a car with aircon is carrying
the extra weight around all year

Mebbe 10-15kg - or about 1% of the average car's kerb weight.

and there must also be a slight extra drag on the engine all year as
the belt to the aircon compressor is turning all the time.

Yes, but when the compressor's clutch is disengaged, it really is very
little extra load.